Abstract: 1) One of the key issues in behavior development is the role which sensory input plays in progressive organization of behavior. Investigations using simultaneous EMG recordings from several muscles in the leg of chick embryos are under way with the intent of studying the emergence of muscle coordination, such as synergistic action and antagonistic inhibition. These EMG will be repeated on embryos in which the precursors of lumbosacral spinal ganglia (neural crest) are eliminated microsurgically. The development of muscle coordination in deafferented and normal embryos will be compared. 2) The development of the cytoarchitecture of the spinal cord of chick embryos, including time of terminal mitosis (birthdates) and migration patterns will be studied, using autoradiography with tritiated thymidine. As a special aspect, the origin of strain-specificity of neuroblasts will be studied by comparing birthdates and migration times of 2 neuroblast types which originate both in the basal plate but have different fates (somato-motor and preganglionic, respectively). 3) One mechanism of control of cell number in nerve centers is initial overproduction and subsequent elimination of a fraction of neuroblasts. To test the hypothesis that degenerating neuroblasts are those which fail to establish peripheral contacts, the peripheral field of primary sensory and motor neuroblasts in the cervical region of the embryonic cord will be enlarged by transplantation of a supernumerary leg bud to that region. The transplant is expected to accomodate additional axons whose perikarya would normally degenerate. Numerical hyperplasia in sensory ganglia and the motor column (compared with the control side) would support the hypothesis.